Thanks for detailed comment! Yes, (unfortunately) you are right. This ‘slight’ size mismatch is killing the mount compatibility.

This is a pipe dream in the best case scenario. It isn’t going to fit. The Hero5 Session is 38mm x 38mm x 36mm, almost a perfect cube.

DJI has this listed as 39mm x 39mm x 22.3mm.

Putting depth to the side for now, you are going from a smaller camera sized at 38mm square to a slightly larger one at 39mm square. Meaning the front face area is 1444 square mm on the Session and 1541 square mm on the DJI 2.
This alone is going to be a problem. If you were going from larger camera to smaller camera (ie, the opposite of what we are doing here) then MAYBE you could make it work with a little bit of added padding or paper on one side of the case.

Once you include depth, you can see why it won’t happen. The Session is 36mm in depth and the DJI 2 is only 22.3 mm in depth.

In the end you have have a larger camera face (it is only larger by 77 mm square but that is a lot more than a little wiggle room) and the body of the DJI 2 is just over 60% the size of the session.

By the time you figure out all the sizes needed to fit it to an old Session case with a piece made in Fusion360, you could have already made a case specifically for the DJI 2 which will always be a better fit.

i guess it should
but it’s much shorter
to use that space with a bigger battery, new mounts would have to be made

Looks good. I guess it fill fit the Session mounts with minimal adaptation or will it not? 39x39mm vs 38x38mm.

https://www.notebookcheck.net/fileadmin/Notebooks/News/_nc3/FANPmwIVIAQhAWS.jpg